Bevy game templates

## What problem does this solve or what need does it fill?

Bevy games are becoming increasingly sophisticated. It's important to see realistic examples for both users and developers: teaching best practices, reducing ramp-up and spotting holes in the engine.

For example, 2D platformers are a very popular starting project, and it can be surprisingly hard to figure out how to make one in Bevy.

Game templates, especially official ones, will help educate users, and ensure that the engine ships with the basic tools to make this easy.

## What solution would you like?

1. Create a templates category on Bevy Assets (or its successor).
2. Create a standardized process by which to make game templates (a meta-template?), likely using `cargo generate`.
3. Create an initial official game template or two. These should likely live within the Bevy org, but in their own repo and should be updated before each official release to spot critical problems.

## What alternative(s) have you considered?

Create more official example games. This is worse because:
- bloats the size of this repo, worsening contributor experience
- cannot be easily used to start new projects
- cannot reasonably rely on 3rd party plugins, even when using them is best practice
- harder to discover
- increases friction with each PR
- harder to delegate authority for

Start by reviewing the Bevy Assets proposal and the suggested cargo generate workflow. Define the template process, create an initial official game template or two in a separate Bevy organization repository, and ensure the templates can be updated before each official release.

Written by the indexing model from the issue text.